Method of electrostatically coating by causing coalescence of coating droplets



Jan. 12, 1960 2,920,982

P. MILLER E. METHOD OF ELECTROSTATICALLY COATING BY CAUSING COALESCENCEOF COATING DROPLETS Original Filed NOV. 5, 1945 2 Sheets-Sheet 1 ii KINVENTOR. E: ENE]? Y R MILLER Jan. 12, 1960 E. P. MILLER 2,920,982

METHOD OF ELECTROSTATICALLY COATING BY CAUSING COALESCENCE OF COATINGDROPLETS Original Filed Nov. 5, 1945 2 Sheets-Sheet 2 05 Gib/ 11: FIG. 6175 INVENTOR EME'RY 1. MIZLER //IlIIIII/Illlll l 'IIIIIIIIIII 1111/ w II I United States Patent =burg Electro-Coating Corp., Indianapolis,Ind., a corporation of Indiana Original application November 5, 1945,Serial No. 626,631, now Patent No. 2,770,210, dated November 13, 1956.Divided and this application July 8, 1955, Serial No. 520,797

6 Claims. (Cl. 117-93) This invention relates to the deposition offinely divided material upon an object in an electrostatic field.

In the manufacture of tinned sheet steel, commonly known as tin plate,used extensively for making soldered seam cans, it is the practice toapply a thin coat of oil to the tinned surface of the tin plate, thiscoating generally being applied to the tinned plate or strip before itis cut into blanks for the forming presses. The coating of oil isdesirable as an aid in separating the sheets from the stack and as anaid in securing proper printing in the subsequent lithographingoperation. An excessive coat of oil on the tin plate interferes with theproper stacking of the blanks by causing the sheets to offset,interferes with the subsequent operations of soldering or lithographing,and has other disadvantages.

It is an object of the present invention to apply to the surface of anarticle, a thin coating of liquid.

Another object is to apply to an article a thin continuous coating ofliquid.

Another object is to apply to a continuous web of material a coatingwhich is maintained substantially uniform over large areas.

Another object is the treatment of an article having spaced droplets orparticles of liquid having dielectric properties on the order ofvegetable, mineral or animal oils thereon to cause spreading orcoalescence of the droplets or particles.

This application is a division of my copending application, SerialNumber 626,631, filed November 5, 1945, now Patent No. 2,770,210.

In accordance with the present invention, a coating is deposited upon asurface in the form of discrete spaced particles of a dielectric liquid,and the coated surface is subjected to an electrostatic field of,sufficient intensity to spread or coalesce the particles. Preferablythe coating is obtained by precipitating liquid particles upon thesurface in an electrostatic ionizing field or zone, and the coatedsurface is exposed to an electrostatic field for a sufiicient length oftime to secure the desired degree of spreading of the particles. Bydepositing the particles upon a moving sheet or article adjacent one endof an ionizing electrostatic field, the field serves the dual functionof precipitating the particles onto the surface and acting on thedeposited particles as the surface moves therethrough to increase orassist spreading and coalescence of the particles.

The suspension of particles preferably is introduced at such velocity asto form an elongated stream or blankdeposited particles will remainunder the coalescing action of the field for a suflicient length of timeto insure adequate spreading or flow.

The spacing of the deposited particles will depend on their distributionin the stream, and the relative movement between the stream and theobject to be coated. When the sheet or object to be coated is moved pastthe stream of particles, the density or thickness of the coat applied isdetermined by the rate at which the material is supplied and the speedof the web or object to be coated.

In the practice of the present invention I am able to obtain acontinuous liquid coat on the surface of the sheet which is thinner thanany continuous coat heretofore obtained by spray methods. This result Ibelieve is in part due to the nature of the stream or blanket of mistwhich in the absence of an electrostatic field, retains its crosssectional shape for some distance; to the substantially uniform andunidirectional defiecting'and depositing action of the electrostaticfield on the mist stream; to the traveling of the web which increasesthe dispersity of the particles deposited; and to the deforming orspreading action of the electrostatic field on deposited particles whichcauses them to spread and to tend to coalesce and form a continuouscoat. The deforming action of the field will vary, depending on thedielectric properties of the liquid particles, their size, the time ofexposure, the strength of the field, and other factors. As is wellknown, vegetable, mineral and animal oils have good dielectricproperties, their dielectric constants ranging from 2.0 to 5.0 and theirresistivity being of the order of 10 to 10 ohms per centimeter cube.Such oils can be readily deformed by the electrostatic field. Aspreviously indicated, the present invention is particularly useful inproducing an exceedingly thin coating of oil on tinplate and such oils,being usually vegetable oils, have dielectric properties of the ordersjust noted. Under comparable conditions of deposit without anelectrostatic field and under the spreading influence of gravity, theparticles would form a non-continuous coating. To obtain a continuouscoat in the absence of an electrostatic field it would be necessary todeposit a sufficiently large number of particles in overlapping relationso that resultant contact between particles would induce subsequent flowto an even film. Such a film would contain more material and thereforewould be thicker than that obtained with the same size particles in anelectrostatic field where a greater dispersity of deposit can be securedand then the spaced droplets can be flattened (i.e. reduced inthickness) and be coalesced by the action of the electrostatic field inaccordance with the present invention. This feature of the invention isof particular value in instances where excessive coating is deleterious.

I prefer to bring the particle suspension into the field through aninduction nozzle opening upwardly and which extends generally in thedirection of movement of the object to be coated. This arrangementemploys the force of gravity to oppose the upward momentum of theparticles introduced into the electrostatic field; assists in minimizingthe agitating effect of ambient air currents; and has the furtheradvantage of avoiding dripping from the nozzle onto the sheet to becoated. In treating sheets of material the induction nozzle ispreferably of about the same width as the sheet to be coated, and thenozzle opening may be made adjustable so that the apparatus is suitablefor treating sheets or webs of various widths. However, I do not intendto exclude the application of a low velocity diverging stream ofatomized liquid in carrying out the invention.

Where an air stream is employed for producing the dispersion it mayserve as a carrier for the dispersed particles, and if desired, this airstream may be augmented by a supplementary air stream. Also it is ofadvantage in some cases to provide an auxiliary air stream through theelectrostatic field which serves as an envelope for the stream ofdispersed particles introduced therein. When employing an exhaust fan orthe like for this purpose, there is provided a sub-atmospheric pressurearea in the coating and spreading zone, which is of benefit inconnection with the introduction of the low velocity nebulized liquidfrom the plenum chamber into the electrostatic field.

The invention is applicable to the coating of webs or objects withvarious kinds of adherent dielectric liquid materials. As examples ofthe liquids which may be applied, there are cited mineral, vegetable oranimal oils, or other liquids having dielectric properties which are ofthe order of these materials.

In particular, when applying a coating of non-drying or slowly dryingoil, I am able to deposit a thinner film in a given time in anelectrostatic field than when the force of gravity or velocity of acarrier air stream is employed as the depositing force Without the aidof an electrostatic field. I believe this is due partly to the atomizingor nebulizing of the liquid in a plenum chamber and introduction of thenebulized liquid into the electrostatic field and partly to the actionof the electrostatic field in pre cipitating and spreading thedielectric oil to cause the deposited particles to merge or coalesce onthe surface to be coated. it thus is possible to cause particles ofdielectric oii to coalesce on a surface in an electrostatic field whichif deposited without an electrostatic field would remain as discretespaced articles. The invention therefore is not limited to simultaneousprecipitation and coalescence, but in its preferred embodiment the twosteps are carried out together.

The web or object to be coated may be of conducting or non-conductingmaterial, and may be of any desired surface texture, although it will beunderstood that the nature of the coating applied will be modifieddepending on the porosity of the web or object, the relative adhering orwetting nature of the web or object and the coating material for eachother; and other factors. Thus, for example, the invention is applicableto the coating of sheet steel with vegetable oil to produce an extremelythin oil coating thereon. Where a non-conducting sheet or object is tobe coated, it will be understood that a conducting backing thereforshould be provided.

The invention will be described in greater detail in connection with theaccompanying drawings wherein a preferred embodiment of the invention isdisclosed by way of example, and wherein,

Figure 1 is a side view of a preferred embodiment of the invention,

Figure 2 is a cross sectional view on line IIII of Figure 1,

Figure 3 is a perspective view with parts broken away of a preferredform of atomizer or disperser,

Figure 4 is a partly diagrammatic view of an air pressure transformerfor converting rotational motion into a proportional gas pressure,

Figure 5 is a fragmentary view of an air valve controller, and

Figure 6 is a diagram for explaining the operation of the controllershown in Figure 5.

Referring to Figure 1, there is shown a framework or housing 1constructed of channel members and sheets or the like, and extendingfrom the side walls 2 and 3 are sets of insulator bars 4 and 5 (Figures1 and 2) which are secured to the side walls 2, 3 in any suitablemanner. The insulator bars support metal electrode frames 8 and 9,respectively, which are suitably spaced apart, and each frame supportsone or more suitable ionizing electrodes 11 and 12, respectively, whichin the preferred embodiment comprise fine wires stretched taut on theframes. However, any suitable construction of ionizing electrodes may beemployed, which are connected by frames 8 and 9 and wires 14 and 15(Figure 2) to one output terminal of a high voltage pack (not shown),the wire 15 passing through insulator bushing 16 in the end wall 17 ofthe housing. The other terminal of the high voltage pack is grounded.There is thus applied to the electrodes a sufficiently high electricalpotential to produce an ionizing discharge. At the top of the housing islocated a hood 18 to which is connected an exhaust duct 19 leading to anexhaust fan (not shown) of any suitable type. The hood is divided fromthe electrostatic field by a distributor or header bafile 21 which has aseries of perforations 22 to distribute the suction action of the hoodover the electrostatic field and thus produce a relatively uniform flowof auxiliary air through the field.

A strip 26 of cleaned tinned steel passes below and part way around agrounded idler roll 27 suitably journalled in the housing 1, and thestrip passes upwardly through the housing between the electrode frames 8and 9, and at the top passes between a driver roll 28 and hold downrolls 29, 3t), then downwardly to a flying shear and roller leveler (notshown). The strip may also be driven by a drive bridle (not shown) inadvance of roller 27.

The apparatus shown is adapted to apply a coating to both sides of thestrip 26 and includes a pair of induction nozzles 35 and 36 on oppositesides of the strip. However, where coating on only one side of the stripis desired, it will be understood that only one induct-ion nozzle isemployed. The induction spouts or nozzles 35 and 36 receive a supply ofdispersed particles from dispersion units 37 and 38 adjustably mountedupon adjustable support bars 41-11 and @2-11 (Figures 1 and 2). Thespouts or nozzles 35, 36 extend in substantially a vertical directionand open upward, so that condensation that occurs within the spoutsdrips back to the oil supply, and no dripping can occur from the spoutsonto the sheet to be coated. As the dispersion units are alike inconstruction only one will be described in detail.

Referring to Figure 3, the dispersion unit 37 comprises a container 41of suitable form, such as a box or tank adapted to contain a liquid.Within the tank are one or more vertical tubes 42., suitably supportedin a bracket 43, which provides a small aperture at its upper end 44 andhas its lower end 45 extending into a sump or trough 46 containingliquid to be atomized. The level of liquid will be maintained at afairly uniform level below the upper end 44 of tube 42, by manual orautomatic replenishing means (not shown), and the space 47 above theliquid level serves as a plenum chamber. An air jet pipe 48 enters thetank 41 and supplies a jet of air at its nozzle end 49 at a suitablepressure across the reduced end 44 of tube 42, whereby a stream ofliquid is sucked up into the air jet and is disintegrated or atomized toform a spray. If desired, a heating element 51 of any suitable type maybe located in an insulated housing 52 below the sump to maintain arelatively uniform temperature therein and thus keep the liquid at arelatively uniform viscosity. Additional insulation of the walls ofchamber 37 may be provided, if desired.

The spray 56 is composed of liquid particles of diverse sizesintermingled with air which imparts to the particles a forward velocity.Preferably, a oaflle 58, which may be curved as shown, supported bybrackets 59 on the wall of the tank is interposed in the path of thespray, so that the larger particles in the spray impact against thebaffle and are disrupted into finer particles, or condense thereon anddrip down into the tank. As a result of the bathing and disruptingaction of baffie 58 and the change of direction of flow of the stream inpassing into the nozzle 35, the particules which are included in the airstream issuing from the induction nozzle 35 are in the form of a linemist or fog. By bringing about atomization of the liquid in a plenumchamber, prior to introduction of the dispersion into the'electrostaticfield, I am able to step down the ther separation of larger particles.vbodiment, the air streams in pipes 48 and 61 receive their supply froma common conduit 65 (Figure 6), and

.being employed in the embodiment shown.

air .velocityissuing into the electrostatic field to reduce oreliminateturbulence therein. The size of the plenum chamber and of thenozzle opening may be varied dependsubstantially non-divergent stream ofthe dispersed particles passing therethrough as indicated by the dot anddash lines 60 (Figure 1) so as to minimize disruption of the Ianenveloping, slowly moving stream of air.

While for certain uses the fog thus produced maybe carried into theinduction nozzles, I prefer to first dilute :the fog by an auxiliary airstream which enters the tank 41 by pipe 61 above the liquid level and ata suflicient velocity to agitateand dilute the fog, and assist in a fur-In the preferred emare proportioned by suitable manually controlledvalves (not shown). Any number of atomizer units desired may becontained within the confining chamber 41, two

The application or induction nozzle 35 preferably is elongated to .aboutthe width of the sheet to be coated, or slightly .wider, and if desired,dampers 66 at each end may be provided, the dampers being manuallypositioned by handles 67 to adjust the width of the fog stream issuingfrom the spouts. These dampers may be locked in posi- :tlOll duringoperation by any suitable means (not shown).

The ionizing zone extends between the ionizing electrodes 11 and 12 andthe sheet 26, and it is preferred to have the openings or mouths of theinduction spouts 35, 36 located within this zone.

The operation of the apparatus so far described now will be explained.The sheet 26 of tinned steel is driven through the apparatus at auniform speed. Streams of oil dispersed in air issue from the spouts 35and 36 at a comparatively low rate of flow and the particles of oiltherein are deflected by the electrostatic field and caused to depositon the moving steel strip. Because of the low velocity of the fog ormist entering the electrostatic field and the substantial absence ofturbulent flow between the end of the spout and the deposition area ofthe moving 5 am able to introduce the fog into the electrostatic fieldat a reduced velocity. As a consequence the electrostatically chargedparticles flow in a curved stream or wall I indicated at 70 (Figure 1)which intersects the strip to be coated near the leading edge of thefield, and the particles coming into contact with the strip are heldthereon by their own wetting action and by the attractive force of thecharge of opposite polarity carried by the strip. Furthermore, since thefog particles are formed from a liquid 'which is a relatively poorconductor, the outer surface of the particles after being deposited willremain charged at the same polarity as the ionizing source, which wouldassist in repelling any floating particles in the field and would causesuch floating particles to be deposited on a portion of the strip notpreviously coated lying between 'or adjacent particles already adheringto the strip. In addition the surface charge upon the free surface ofsuch particles will cause the free surface to be attracted to the sheetand thus the droplet will be spread so long as this charge ismaintained. Inasmuch as the particles are deposited in spacedrelationship at the leading edge of the field, they are maintained inthe field and subject to its action for a period of time after beingdeposited. This prolonged exposure to the field assists in maintainingthe charge on the exposed surface of the deposited particles until theflattening and spreading action is complete. Furthermore, the action ofthe electrostatic field in lowering' the surface tension of the liquidparticles deposited on the strip assists in causing relatively poorwetting liquids or liquids of relatively good dielectric properties toadhere to and spread over and coalesce on the surface of the strip. Thusthere is obtained an extremely thin coating of oil on the tinned strip.In explaining the theory of operation of the apparatus and method, I donot Wish to be confined to such theory in claiming my invention.

While ordinarily the strip travels at a uniform rate through the coatingmachine, there are times, for example, during the automatic welding ofthe ends of two successive strips in the continuous type strip steelmill, when the travel speed is reduced by about one half. Also whenstarting or stopping the operation, there is a gradual deceleration oracceleration of the strip. In the apparatus disclosed, I regulate orcontrol the volume-rate of flow of the fog blanket issuing from theinduction nozzle proportionately with the lineal speed of the steel web,so that a relatively uniform deposition of the particles will be maderegardless of variations in the rate of travel of the steel strip. Themechanism by which this is accomplished now will be described.

Referring to Figure 4 there is partly diagrammatically shown aTransometer which comprises a vertical shaft carrying a plate 76 whichis rotatably driven by horizontal shaft 77 in any suitable manner, andwhich in turn is suitably driven by a part of the drive mechanism forsheet 26, such as the drive bridle. Plate 76 carries two yokes 81 and 82in which are pivoted two rocker arms 83 and 84 carrying weights 85 and86 at their ends. A push rod 87 is suitably supported in the housing 75and at its lower end carries a collar 88 adapted to be engaged by theends of rocker arms 83 and 84. At the top of the housing, a diaphragm 90is suitably secured by a cover 91 to provide a pressure chamber 92 witha port 93 having a conduit 94 connectedzthereto. Port 93 connects with apassage 95 through the clamped periphery ofthe diaphragm and terminatesin an orifice 96. A nozzle 97 is disposed opposite the orifice 96 and isconnected by a suitable flexible coupling 98 to a conduit 99 whichsupplies to the nozzle a stream of air at a constant pressure. As thisapparatus is commercially available a more detailed description of itsconstruction is not required.

The operation of the apparatus so far described in connection withFigure 4 now will be explained. At a given sheet speed air issuing fromnozzle 97 will enter orifice 96 and through passage 95 will develop apressure in chamber 92 and conduit 94. Nozzle 97 will assume a positionrelative to orifice 96 such that the force of the air pressure inchamber 92 on diaphragm 90 counterbalances the upward thrust on rod 87due to centrifugal force on weights 85, 86. The pressure in chamber 92is at all times proportional to the speed of theshaft 77 or strip 26(Figure 1) and therefore the pressure supplied to conduit 94 is alsoproportional to the speed of strip 26.

The pressure in conduit 94 is transmitted to the chamber 101 (Figures 5and 6) under a diaphragm 102 contained within a housing 103, a spring104 exerting a bias on the diaphragm. A push rod 105 engaging diaphragm102 engages a lever 106 pivoted at 107, which is urged against a bearingmember 108. A threaded rod 109 engages the bearing 108 and by means of ahand wheel 110 thereon, the bearing member can be moved toward or awayfrom the pivot 107 to vary the mechanical advantage of lever 106.

As previously mentioned, the conduit 65 supplies air to the atomizingdevice illustrated in detail in Figure 3. This conduit contains a valveindicated generally at 115 which embodies a seat or orifice 116 (Figureand a valve closure member 117, the seat and closure being of such formthat the flow through the seat is proportional to the position of theclosure. The closure is secured to a stem 11% passing through a sealingdevice 119 of suitable construction and abutting a diaphragm 121 heldbetween the housing 122 and a cover 123. A spring 124 urges thediaphragm 121 upward and urges valve 117 into closed position. A conduit125 supplies a constant pressure flow which is conducted by a conduit126 to pressure chamber 127 and tends to oppose spring 124 to open valve115.

The housing 128 which receives the conduit 125 pro- Vides a bleedorifice 131 (Figure 6) by which air is bled to the atmosphere and aclosure 132 secured to lever 133 pivoted at 134- controls the bleedtherethrough. Lever 133 and closure 132 are urged to open position by aspring 135, and a spring ferrule 136 is secured to a bar 137' which inturn is secured to valve rod 118. The lever 133 is urged by spring 135into engagement with heating member 168. A finger wheel 138 permitsmanual adjustment of spring 135.

The operation of the control apparatus now will be described. Assumingthe sheet '26 is moving at a constant speed, the pressure in chamber 92(Figure 4) and in conduit 94 is constant and is proportional to thespeed of sheet 26. Under influence of the pressure in chamber 101conducted thereto by conduit 94, the diaphragm 102 will take up aposition of balance determined by the pressure in chamber 101 opposed bythe bias of spring 135 which in turn depends on the position of valverod 118 and valve 115. The position of closure 132 relative to orifice131 will similarly be established by these two conditions. Underconditions of fixed bleed at orifice 131 the pressure in chamber 127will determine the position of valve 115 and thus determine the rate offlow of air through conduit 65 to the pipes 61 and 48. When the sheet 26assumes a lower speed, the change in centrifugal force on weights 85 and86 causes them to move inward and allows nozzle "97 to take up a newposition of balance with orifice 96 so that the pressure in chamber 92,conduit 94 and chamber 101 drops to a different value. This allowsspring 135 to pivot lever 133 downward to move closure 132 further fromorifice 131 to allow a greater bleed therethrough, which results in areduction of loading pressure in chamber 127 and a further restrictionof valve 115 to reduce the flow of air through conduit 65 to theatomizing device. Valve 115 under such conditions will reach a newposition of balance in which the flow of air to the atomizing device 37is proportional to the new speed of sheet 26. When speed of sheet 26increases, an action reverse to that above described will againestablish the proper relation between the speed of the sheet and theflow of air to the atomizing device. By correlating the flow of air tothe atomizing device with the speed of the sheet, the flow of atomizedmaten'al issuing from nozzle 35 is likewise correlated or proportionedto the sheet speed to obtain a consistently uniform deposit of materialindependent of the sheet speed.

Various modifications may be made in the invention described withoutdeparting from the spirit or scope thereof.

I claim:

1. The method of producing an extremely thin coating on an article whichcomprises depositing droplets of a liquid having a dielectric constantof at least 2.0 and a resistivity of the order of at least ohms percentimeter cube in spaced relationship on the article, creating anelectrostatic field over the surface of the article carrying thedroplets to maintain on the exposed surfaces of the droplets an electriccharge, and maintaining the charged droplets in said field until theyhave been flattened by said charge and thereby coalesced into acontinuous film over said article surface, said droplets being depositedin number insuflicient to coalesce and form a continuous film in theabsence of said field.

2. The method of producing an extremely thin coating on an article whichcomprises introducing into a treating zone the article carrying on itssurface spaced droplets of a liquid having a dielectric constant of atleast 2.0 and a resistivity of the order of at least 10 ohms percentimeter cube, creating an electrostatic field over said surface ofthe article in the treating zone to accumulate on the exposed surfacesof the droplets an electric charge, and maintaining the charged dropletsin said field until they have been flattened by said charge and therebycoalesced into a continuous film over said article.

3. The method of producing on an article surface an extremely thincoating of oil having a dielectric constant of at least 2.0 and aresistivity of the order of at least 10 ohms per centimeter cube, whichcomprises moving the article along a predtermined path, creating betweenthe article surface and an electrode positioned adjacent said path anelectrostatic field extending for a substantial distance along saidpath, depositing on said article surface adjacent the point where saidsurface enters said field spaced droplets of said oil, the exposure ofsaid droplets to said field causing the accumulation on the exposedsurfaces of the droplets of an electric charge sufficient to flatten andcoalesce the droplets into a continuous film, said droplets beingdeposited in number insufficient to coalesce and form a continuous filmin the absence of said field.

4. The method of producing on an article surface an extremely thincoating of oil having a dielectric constant of from 2.0 to 5.0 and aresistivity of the order of 10 to 10 ohms per centimeter cube, whichcomprises moving the article along a predetermined path, creating anelectrostatic field extending between an electrode positioned adjacentsaid path and extending parallel thereto for an appreciable distance andthe article surface, introducing the oil in atomized form into saidfield adjacent the point where said surface enters said fieldelectrostatically to charge the atomized particles and therebyelectrostatically to deposit said particles in spaced relationship onsaid surface, continuing to move said article through said field tomaintain on the exposed surface portions of the deposited particles anelectric charge sufficient to flatten the particles and coalesce theminto a continuous film, the movement of the article through said fieldbeing so rapid that the number of particles deposited is low enough thatthey would not coalesce and form a continuous film in the absence of theelectrostatic forces acting on their charged surface portions.

5. The method of applying a thin liquid coating to the surface of anarticle, which comprises creating an ionizing electrostatic field overthe article surface, introducing finely divided particles of a liquidinto said field to be electrically charged and electrostaticallydeposited in spaced relation on the article surface by the action of thefield, said liquid having a dielectric constant of the order of at least2.0 and a resistivity of the order of at leastll) ohms per centimetercube to insure the retention of electric charges on the exposed surfacesof the particles for an appreciable time following their deposition andmaintaining the deposited particles in said field to maintain saidcharges on said exposed surfaces whereby the electrostatic forcesexisting as a result of the surface charges will flatten the depositedparticles.

6. The method of applying a thin liquid coating to the surface of anarticle, comprising the steps of creating a cloud of fine particles of adielectric liquid bearing electrical charges of similar sign, moving thearticle to be coated through such cloud, while maintaining the articleat a particle-attracting potential electrostatically to deposit thecharged particles in spaced relation on the surface of the article as itmoves through the cloud,

Q said liquid having a dielectric constant of the order of from 2.0 to5.0 and a resistivity of the order of 10 to 10 ohms per centimeter cubeto insure the retention of electric charges on the exposed surfaces ofthe particles for an appreciable time following their deposition, andsubjecting the deposited particles while still liquid and while theirexposed surface portions still retain the electric charge, to the actionof an electrostatic field maintained over the article surface, thepotential-gradient of said field being in such diretcion as to tend tocause the exposed, electrically charged surfaces of the particles to beelectrostatically urged toward the article surface and the particles tobe thereby flattened and coalesced to form a continuous film, the rateof movement of the article through the cloud being so rapid that thenumber of particles deposited is low enough that they would not 10coalesce and form a continuous film in the absence of the electrostaticforces acting on their charged surface portions.

References Cited in the file of this patent UNITED STATES PATENTS1,855,869 Pugh Apr. 26, 1932 2,191,827 Benner Feb. 27, 1940 2,221,338Wintermute Nov. 12, 1940 2,334,648 Ransburg Nov. 16, 1943 2,437,606Kaufman Mar. 9, 1948 2,447,664 Pegg Aug. 24, 1948 2,608,176 Jenkins Aug.26, 1952 2,666,716 Kadell Jan. 19, 1954 2,710,589 Brunner June 14, 1955

1. THE METHOD OF PRODUCING AN EXTREMITY THIN COATING ON AN ARTICLE WHICH COMPRISES DEPOSITING DROPLETS OF A LIQUID HAVING A DIELECTRIC CONSTANT OF AT LEAST 2.0 AND A RESISTIVITY OF THE ORDER OF AT LEAST 10**12 OHMS PER CENTIMETER CUBE IN SPACED RELATIONSHIP ON THE ARTICLE, CREATING AN ELECTROSTATIC FIELD OVER THE SURFACE OF THE ARTICLE CARRYING THE DROPLETS TO MAINTAIN ON THE EXPOSED SURFACES OF THE DROPLETS AN ELECTRIC CHARGE, AND MAINTAINING THE CHARGED DROPLETS IN SAID FIELD UNTIL THEY HAVE BEEN FLATTENED BY SAID CHARGE AND THEREBY COALESCED INTO A CONTINUOUS FILM OVER SAID ARTICLE SURFACE, SAID DROPLETS BEING DEPOSITED IN NUMBER INSUFFICIENT TO COALESCE AND FORM A CONTINUOUS FILM IN THE ABSENCE OF SAID FIELD. 